This invention relates to a process for making expandable styrenic polymers anti-static and anti-lumping during pre-expansion and after molding by coating certain quaternary ammonium salts onto the expandable polymer particles or the molded articles.
The making of low density, cellular, shaped, plastic articles from expandable particles of styrene polymers is well known. Such particles generally contain a blowing agent which boils below the softening point of the polymer and which will cause the particles to expand when they are heated. When the expanded particles are heated in a mold cavity, the particles expand further to fill the mold and fuse together to form a shaped article.
The formation of molded articles from expandable styrene polymer particles is effected in two steps: (1) pre-expanding of foamable styrene polymer particles to a density of between 1 and 3 pounds per cubic foot; and (2) further heating the pre-expanded particles in a closed mold to cause further expansion and form a fused, one piece article having the shape of the mold. The second step is what is normally referred to as "molding".
The pre-expansion step may be carried out by heating the expandable polymer particles by any suitable heating medium such as steam, hot air, hot water, or radiant heat. A widely used method of pre-expansion of the particles is a process such as that disclosed in U.S. Pat. No. 3,023,175 by Rodman. Another method is that disclosed in U.S. Pat. No. 3,577,360 by Immel, which teaches the pre-expansion of polymer beads by heating in a dry atmosphere followed by the application of a vacuum to cause the articles to expand to the desired density.
An undesirable result of the pre-expansion before introduction into the mold is the tendency of the pre-expanded particles to clump together and form lumps which render the particles unsuitable for molding. These lumps cannot be properly conveyed in processing plants and are unsuitable for charging into molds of intricate contour due to improper fill-out of the molded articles. Even if reasonable fill-out is achieved, variations in density within the molded articles can result and voids also occur.
Various methods have been proposed for the prevention of lumping during pre-expansion, but these have all suffered from some objectionable feature such as rapid loss of blowing agent, poor flow of the treated beads, dust problems which cause plugging of the steam ports, and often serious reduction in fusion of the particles on molding. U.S. Pat. No. 3,520,833 teaches the addition of lecithin during the impregnation of the particles with the blowing agent. Unfortunately, the lecithin imparts an undesirable odor to the molded articles. U.S. Pat. No. 3,462,293 teaches to coat the particles with polymeric materials by a fluid bed process. This process involves an additional expense of fluidizing the particles and coating with the polymer latexes. U.S. Pat. No. 3,444,104 teaches the addition of calcium silico aluminate. This additive tends to foul transfer lines and molds with clay-like deposits.
Another problem encountered with styrenic polymer particles is that of static charge on the particles, both before and after pre-expansion. The static charges cause problems in conveying the particles smoothly into, for example, the pre-expander or during the molding operation. Another area where static charges become a major problem is in the final molded product. Thus, molded styrene polymer used to package electronic equipment can damage the enclosed equipment due to static discharge. Various antistatic agents have been used to attempt to overcome the static problems.